What does the 'M' and the 'T' stand for on the ground track / heading indicator?

'M' is displayed for Magnetic and 'T' is indicated for True.

'M' (Magnetic) will show only when you are moving and when you are in an area that is less than 63.5 latitude and greater than -65 Latitude (non-polar regions).

Image 1:  Heading indicator with an 'M' showing (taken over Wyoming).

 

'T' (true) is displayed on the ground track indicator whenever you are stationary or if you are moving while in an area that is greater than 63.5 latitude or less than -65 latitude (polar regions). 

Image 2:  Heading indicator with a 'T' showing at far north latitude (taken over the Northwest Territories in Canada).